Ppl naturally want to put something in the Overview backdrop, like a blurred version of the wallpaper (see how popular Blur My Shell is). Orthogonally, some ppl want the wallpaper to be static, rather than move with workspaces (it can be distracting).
I'm not prepared to dive head-first into image rendering and blur shaders yet, so I made it possible to put a layer surface into the backdrop. This enables both of those cases, likely even more cool things!
https://github.com/YaLTeR/niri/wiki/Overview#backdrop-customization